1. Field of the Invention
The invention relates to a method for resolving ambiguity in the determination of antenna angle of view and Doppler frequency in synthetic aperture radar.
2. Description of the Prior Art
Synthetic aperture radar (SAR) is an active microwave imaging method. A radar transmitting-receiving means usually carried by an aircraft or a satellite coherently records the echoes of high-frequency pulses transmitted at the rate of the pulse repetition frequency (PRF). Usually, the antenna centre axis is aligned approximately perpendicularly to the flight path.
The radar echoes are coherently demodulated, digitized and stored as a complex valued matrix u(i, k), the dimension i standing for distance or "range" and k for flight direction or "azimuth". This socalled raw data can be converted with the aid of a two-dimensional correlation to a highly resolved image of the illuminated area, this also being referred to as "focussing" or "compressing".
In the distance or range direction the radar principle is employed, i.e. determination of the echo travelling time. In the azimuth direction a correlation is performed with a function having the phase history of the echoes of a point scatterer. This is a socalled chirp function, i.e. a function with linearly varying instantaneous frequency with the two-way azimuth antenna pattern as envelope curve. This chirp function can be adequately described by the parameters "FM rate" and "Doppler centroid". The Doppler centroid f.sub.DC is the instantaneous frequency at the maximum of the envelope curve. In the case of aircraft SAR and with an antenna centre axis aligned exactly perpendicular to the flight path the Doppler centroid f.sub.DC is equal to zero. If the centre axis however differs from said direction by a socalled squint angle .phi., then: ##EQU1## where v denotes the flight velocity and .lambda. the radar wavelength.
In the case of satellite-supported SAR the rotation of the earth additionally manifests itself in a squint angle between the antenna angle of view and the effective flight path projected onto the earth.
The parameter f.sub.DC is absolutely essential for constructing the azimuth correlation kernal for an image focussing. An erroneous Doppler centroid f.sub.DC leads to deterioration of the the resolution and the signal to noise ratio, to ghost images and to a geometrical distortion.
In the case of many SAR sensors the squint angle .phi. cannot be determined from the sensor attitude and position data accurately enough to satisfy the requirements of the Doppler centroid f.sub.DC accuracy. This applies in particular to high-frequency SARs, for example in the X band, and to relatively unstable sensor platforms, such as the Space Shuttle. In such cases the parameter f.sub.DC must be determined from the radar echoes themselves.
To determine the angle of view of the antenna the effect is utilized that the antenna pattern is found as the envelope of the azimuth power spectrum. As a result, all f.sub.DC estimators are based on the azimuth spectral analysis of the radar data. Since however due to the specific imaging behaviour the SAR signal is sampled in the azimuth direction with the pulse repetition frequency (PRF), a periodic repetition of the azimuth spectrum results. Such methods lead fundamentally to an ambiguity regarding the absolute position of the Doppler centroid f.sub.Dc, and this can be expressed as follows: EQU f.sub.DC =f.sub.DC +p.multidot.PRF (2)
where f.sub.DC is an estimated Doppler centroid in the base band [-PRF/2,+PRF/2], and p is an integer ambiguity number. For technical reasons the pulse repetition frequency (PRF) is usually chosen so low that the parameter f.sub.DC can in fact lie in various PRF bands.
Two methods are known for resolving Doppler frequency ambiguity. In the socalled multiple PRF technique as described by F. K. Li and W. T. K. Johnson in the article "Ambiguities in Spaceborne Synthetic Aperture Radar Systems", IEEE Trans. on Aerospace and Electronic Systems, Vol. AES-19(3), pages 389-397, 1983, at the start and end of the data acquisition the SAR sensor is operated with different pulse repetition frequencies. Depending on the absolute position of the Doppler centroid f.sub.DC this leads to different f.sub.DC values, from which the correct PRF band can be determined. Disadvantages of this method reside inter alia in that it must be operated already at the data acquisition. and that a wide range of pulse repetition frequencies must be implemented. Moreover, with this method the PRF ambiguity can only be resolved for raw data in which a multiple PRF sequence was activated.
A second method based solely on an analysis of the raw data is the socalled look correlation technique as described for example by A. P. Luscombe in the article "Auxiliary Data Networks for Satellite Synthetic Aperture Radar", in Marconi Review, Vol. XLV, No. 225, 1982, or also by F. G. Cumming, P. F. Kavanagh and M. R. Ito in an article "Resolving the Doppler Ambiguity for Spaceborne Synthetic Aperture Radar" in the Proceedings of IGARSS'86, pages 1639-1643, Zurich, Ref. ESA SP-254, 1986. However, the accuracy of this method drops with the square of the radar frequency and said method is therefore not very suitable for high-frequency SARS. The computational load is considerable because full focussing of the data is necessary; moreover, this method fails with scenes of low image contrast.